In general a heat pump system includes a reversing valve to change the connection from the compressor discharge line to the indoor coil or the outdoor coil, to change the respective functions of these coils from a condenser to an evaporator, and from an evaporator to a condenser. As the heat pumps have received greater acceptance, efforts have been made to improve their control systems. One problem frequently encountered has been associated with the accumulation of ice on the outdoor coil when the outside air temperature is near or below freezing. When this occurs it has been a common practice to activate the reversing valve and remove heat from the system, "dumping" the heat into the outdoor coil to melt the ice. The equipment has usually been left in this defrost mode until the coil temperature reached 75.degree., or some suitable temperature, to indicate an effective defrost operation had taken place. However if the wind is blowing, it may cool the outdoor coil and "fool" the system. Thus the system would stay in the defrost mode for hours or days, rendering the system ineffective at lower temperatures.
It is therefore a principal object of this invention to provide a heat pump control system which provides an effective defrost cycle operation, without staying in the defrost cycle for an extended time period.
Another important object of the invention is to provide such a heat pump control system in which the "balance point" temperature (below which the heat pump cannot satisfy the call for heating, and supplemental heat must be provided) can be simply set by the installer and/or the ultimate user of the system.
Another salient object of the invention is to provide such a heat pump control system which, when added to an existing heating system, is simply modified to move up the standby-heat temperature setting, below which the compressor is prevented from operating and standby heat is added to the supplemental heat.